Human monocytes obtained by counter-current centrifugal elutriation released arachidonic acid (AA) when challenged in vitro with concanavalin A (Con A), as well as with other soluble (phorbol myristate acetate [PMA] or ionomycin) or particulate stimuli (serum-treated zymosan (STZ)). 5-Lipoxygenase (5-LO) products, such as leukotriene B4 (LTB4) were conspicuously absent. Release of arachidonate and its metabolites in response to Con A was dependent on the presence of extracellular Ca2+, but not Mg2+. In contrast to serum-treated zymosan (STZ) challenge, which resulted in increased inositol trisphosphate and LTB4 release, Con A-induced inositol phospholipid hydrolysis in monocytes was limited to phosphatidylinositol or phosphatidylinositol monophosphate. Despite an inability to augment LTB4 release, Con A or PMA induced a loss of 5- lipoxygenase from a cytosolic compartment which was similar to that achieved with a calcium ionophore (ionomycin), a potent stimulus for LTB4 generation. When cell-associated LTB4 was evaluated, evidence for increased LTB4 production was obtained in response to either stimulus (PMA>Con A). In combination, however, PMA and Con A treatment resulted in monocyte LTB4 release comparable to that observed with the calcium ionophore or STZ. LTB4 release in response to all stimuli tested was inhibited by MK-886, a drug which binds to 5-lipoxygenase-activating protein (FLAP).